Ulex europaeus, a native of Western Europe and the Mediterranean coast, is a serious agricultural and environmental weed in many parts of the earth, including Australia. Tetranychus lintearius attacks U. europaeus in its natural range and is one of the four biological control agents introduced into Australia to regulate populations of U. europaeus. Tetranychus lintearius is specific to different species of Ulex, which enhances its status as a key biological control agent. Little is known on the physiological effects and structural changes induced by T. lintearius on U. europaeus. This article describes anatomical changes and evaluates the physiological effects in U. europaeus consequent to feeding by T. lintearius for more than 100 d. This study demonstrates that feeding by T. lintearius reduces the quantity of photosynthetic tissue in U. europaeus, as a result of feeding on mesophyll parenchyma by inserting its stylets through the stomatal aperture. Unfed parenchyma cells display signs of stress, either ...